Electroacoustic transducer with improved electromagnetic drive



Jan. 9, 1968 ss JR 3,363,227

ELECTROACOUSTIC TRANSDUCER WITH IMPROVED ELECTROMAGNETIC DRIVE FiledFeb. 1, 1966 2 Sheets-Sheet 1 FIG! INVENTOR liigflw MAssA, JR.

ORNEY Jan. 9, 1968 F. MASSA, JR 3,363,227

ELECTROAGOUSTIC TRANSDUCER WITH IMPROVED ELECTROMAGNETIC DRIVE FiledFeb. 1, 1966 2 Sheets-Sheet 2 FIGZ IN'VENTOR RANK MASSA,JR.

TTORNEY United States Patent OflFice 3,363,227 Patented Jan. 9, 1968Mass.

Filed Feb. 1, 1966, Ser. No. 524,013 8 Claims. (Cl. 340--8) Thisinvention is concerned with an improved electromagnetic transducer, and,more particularly, with an improved electromagnetic design which isparticularly advantageous for efficient operation of electroacoustictransducers operating in the mid and high audible frequency regions.

It is well known that electromagnetic circuits which must operateefficiently at the higher frequencies must have magnetically-conductingelements which employ thin laminations in order to reduce theeddy-current losses in the magnetic core. The lamination thickness mustbe particularly reduced if a high operating flux density is required inthe magnetic circuit. The eddy-current losses are proportional to t f Bwhere t is the lamination thickness, is the frequency, and B is the peakvalue of the alternating flux density. It can be seen readily from theabove relationship that as the frequency and flux density increase in amagnetic circuit, the thickness of lamination must be reduced in orderto hold the eddy-current losses to reasonable levels so that theefiiciency of the electroacoustic transducer can be kept at anacceptable level.

The conventional method of laminating a magnetic circ'uit consists inemploying stacks of thin magnetic stampings which are held together by abonding cement or by bolts or clamps. For electromagnetic circuitsoperating at the lower audio frequencies the lamination thickness may beof the order of and the problem of stacking fiat laminations of suchthickness presents no difiiculty in the handling and assembling of thestampings. When laminations have to be used in which the thickness isreduced to the order of only a few thousandths of an inch or less thecost of handling many thousands of paper-thin laminations becomes veryhigh. My invention solves this problem with a new design of anelectromagnetic transducer for eflicient high-power operation, whichemploys large quantities of thin magnetic lamination material withoutthe necessity of handling individual thin stampings.

It is a primary object of my invention to produce a laminatedelectromagnetic circuit for use in operating an electroacoustictransducer which eliminates the need for stacking and handling largequantities of very thin stampings such as are required in conventionaldesigns.

It is another object of my invention to increase the electromagneticelficiency of an electromagnetic trans ducer by employing a design ofmagnetic structure and associated current coils such that 100% of theconductor used in the coils is actively linked with the magneticcircuit.

A still further object of my invention is to improve the method ofassembling an electromagnetic circuit employing the permanent magnetsfor polarization to achieve accurate control and maintenance of auniform air gap even when the dimensions of the structure over which theair gap is to be maintained are very large.

Another object of my invention is to produce an improved electromagnetictransducer employing an inertial type of electromagnetic drive whereby100% of the electrical and magnetic materials are closely coupled forcreating increased electromagnetic driving forces whereby the efficiencyof the transducer is increased over conventional designs.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The invention,itself, however, both as to its organization and method of operation, aswell as its advantages thereof, will best be understood from thefollowing description of several embodiments thereof when read inconnection with the accompanying drawings in which:

FIG. 1 is a cross-sectional view taken in a plane containing the centerline of one type of transducer illustrating the embodiment of myinvention;

FIG. 2 is a sectional view taken along line IIII of FIG. 1, and

FIG. 3 is a view taken along the line IIIIII of FIG. 1.

Referring to the drawings, reference numeral 10 generally designates anelectroacoustic transducer employing an electromagnetic drive systemconstructed in accordance with this invention. The transducer 10comprises rigid circular plate 11 having a pair of parallel planesurfaces. Both sides of plate -11 are counter-bored, as illustrated, toprovide two recessed sections with an intervening annular web portion12. A hole is shown through the center of web portion 12, the purpose ofwhich is only to eliminate unnecessary weight from the vibratingstructure. Two tightly-wound annular assemblies 13 and 14 of thinmagnetically-conducting ribbon are securely bonded to the opposite facesof the web member 12 such that plate member 11 becomes an integratedrigid structure in combination with magnetic assemblies 13 and 14.Magnetic assemblies 13 and 14 are substantially identical in theconstruction as illustrated in FIG. 1. Each assembly is so formed as toprovide an E-shaped magnetic cross section.

One method which I have found very satisfactory for preparing each ofthe assemblies -13 and 14 is to employ a circular piece of rigid tubing15 as a form which is attached to a lathe or other similar coil-windingequipment. A roll of continuous thin magnetic ribbon or tape,illustrated in cross-section by the reference character 16, is woundtightly over the form 15 using an adhesive such as epoxy betweensuccessive layers as the thin magnetic ribbon or tape 16 is wound overthe form 15. When the desired thickness of the wound magnetic tape isachieved, the full Width tape 16 is replaced by a narrower magnetic tape17 and the winding continued until the desired thickness of the narrowtape 17 is built up, as illustrated. Next, a ring-shaped form is placedadjacent to the wound narrow section 17 to fill the space between thenarrow ta e 17 and the full width tape 16 to provide a continuoussurface for continuing the winding of the wide magnetic ribbon or tape16 to provide a center portion 18 of the magnetic E-shaped assemblywhich is shown in cross-section in FIG. 1. After the center section isbuilt up to the required amount, a narrow ribbon or tape 19 is used tocontinue the formation of the second annular slot in the E- shapedcross-section, and, finally, a second ring-shaped form is inserted toprovide a continuous surface for winding a final section 20 of wideribbon to complete the fabrication of the magnetic assembly as shown inFIG. 1.

By followng the process just outlined, a composite circular magneticelement is fabricated which, upon removal of the two ring-shaped forms,becomes a rigid circular annular laminated assembly with twocircumferential slots, as illustrated by the E-shaped cross-sectionalview of the assemblies 13 and 14 in FIG. 1. By employing thisfabrication procedure, I am able to use a continuous strip of very thinmagnetic material to produce large-size magnetic assemblies forefiicient high-frequency operation with low production cost as comparedwith the alternative method of using E-shaped stampings for the magneticstructure. Another advantage of my circular ring-shaped magneticassembly is that is creates a continuous circular slot in the magneticstructure within which circular coils 21 and 22 may be assembled. Inthis design, the currentcarrying coils 21 and 22 are completelysurrounded by active magnetic material, which means that 100% of thewinding is effective in generating electromagnetic forces. In theconventional assembly employing flat stacks of E- shaped laminations,rectangular-shaped coils are dropped in the parallel slots which areformed by the flat laminations and the ends of the coils remain outsidethe magnetic structure, and serve no useful electromagnetic function.The over-hanging portions of the rectangular coils add unnecessarymoving mass to the vibrating system and also add unnecessary electricalresistance to the coils, which increases the electrical losses duringthe operation of the transducer.

In order to obtain the highest possible amount of actlve electricalconductor in the two circular slots of magnetic assemblies 13 and 14, Ihave chosen to wind the coils with insulated copper ribbon in much thesame manner as I have described in connection with the fabrication ofthe laminated magnetic structures. By employing thin insulated copperribbon for the coils 21 and 22, they can be wound to very accuratetolerances, and the copper will occupy practically the fullcross-sectional area of the circular slot when it is assembled in place.In winding the coils, the first and last turn may be folded at 90 sothat the ends of the copper ribbon will project out through the openends of the slots. These free coil ends may be again folded over andcemented in place into radially under-cut recesses in the faces of themagnetic ring assemblies 13 and 14 so that the coil ends may be broughtout into the center opening of the plate member 11 without projectinginto the air gap as the leads run by over the face of the magneticassembly. The physical details for recessing a small radial slot in thefaces of the magnetic assemblies 13 and 14 are not shown since they arenot important in connection with the main purpose of this invention.Rather than complicate the illustrations and figures, the coils 21 and22 are schematically shown connected together in series by theelectrical conductors 23 and 24, and the complete inter-connected systemof coils is in turn connected by means of electrical conductors 25 and26 to the insulated electrical terminals 28. Electrical terminals 28 aresealed into the counter-bored region of a housing 30 as shown. Arubber-covered underwater cable 31 containing two insulated conductors32 is molded to a metallic flanged member 33 which contains a machinedgroove with a conventional O-ring 34 to effect an underwater seal whenflanged member 33 is bolted in position to the housing 30 by means ofbolts 35. The conductors 32 are soldered to the electrical terminals 28to establish electrical connection from an external power source intothe current coils 21 and 22 for operating the transducer.

An important feature of my invention is that I achieve very highelectromagnetic efficiency by the design which has been described. Byusing two electromagnetic assemblies, one on each side of plate 11, Ieffectively double the electromagnetic drive forces for operating thetransducer which is desirable for high-power underwater operation.

To complete the operating electromagnetic circuit for the illustratedtransducer, I prepare a pair of magnetic assemblies 37 and 38 to matewith the assemblies 13 and 14 previously described. In preparing themagnetic assemblies 37 and 38, I advantageously employ a rigid tubularmember 39 to serve as a form in the same manner as member 15 is employedin the assemblies 13 and 14. Over the form 39 a tight continuous windingof thin magnetic ribbon is rigidly bonded to build up a section 40having a thickness which is identical to the corresponding thickness ofthe mating section 16 or the magnetic assembly 13 or 14. At this pointthe winding of the thin magnetic ribbon is in terrupted and a number ofpermanent magnets 41, shown both in FIGS. 1 and 2 and having acylindrical contour, are bonded to the outer periphery of the fabricatedsection of the wound magnetic tape. The magnetic ribbon is then bondedto an outer surface portion 42 of the magnets 41, and the winding iscontinued until the desired thickness of magnetic material is achievedto provide a section 43 corresponding to the section 18. At this pointthe winding is again interrupted and a second set of mag nets 44 isbonded into position, as illustrated in FIGS. 1 and 2. The cylindricalcontour of the surfaces of the magnets 44 corresponds to the diameter ofthe built-up magnetic winding. After assembling the magnets 14 intoposition the magnetic ribbon is tightly bonded and wound over the outersurface 45 of the magnet 44 until the thickness of the winding is builtup to the desired amount to provide a section 46 mating with the section20 to complete the total magnetic assemblies 37 and 33.

It is noted that the wound coil assemblies can have shapes other thanthe illustrated cylindrical shape, such as oval, square, rectangular orother polygonal shapes, in which case the magnets should havecorresponding shapes. It is also noted that an electromagnet assemblycan be used in place of a permanent magnet assembly in which case theassemblies 37 and 38 have constructions similar to that of theassemblies 13 and 14.

In the transducer which I have used to illustrate one application of myinvention, the operation of the electromagnetic transducer results fromthe inertial alternating forces generated between a massivespring-suspended base member which contains a portion of theelectromagnetic circuit and the driven portion of the magnetic circuitwhich is attached to the vibratory sound radiating portion of thetransducer. Having prepared the lamination and permanent magnetassemblies 37 and 38 as rigid composite cylindrical annular units, Isecurely bond the units onto planar recessed surfaces 47 and 48 ofmassive base members 49 and 50. The base members 49 and 50 arefabricated of a non-magnetic material such as bronze so as not tomagnetically short-circuit the permanent magnets 41 and 44. Beforebonding the magnetic assemblies 37 and 38 to the recessed surfaces 47and 48 of parts 49 and 50, I prepare the end surfaces of the magneticassemblies 37 and 38 by grinding or by other suitable means to make themperfect planes. The same surface preparation is preferably employed forthe assembled magnetic structures 13 and 14 and prior to bonding theirsurfaces into the recessed sections of plate 11.

To secure the desired operation of the transducer illustrated in FIG. 1,a plurality of peripherally-mounted spring members 51 and 52 attached bymeans of bolts 53 and 54 to the prepared outer peripheral surfaceportions of base members 49 and 50, as shown. The spring members 51 and52 preferably have intermediate portions of sufficient flexibility topermit resilient movement of the ends thereof toward and away from eachother. A layer of epoxy cement may be advantageously employed betweenthe faces of the spring members 51 and 52 and the surfaces of basemembers 49 and 50 at the time of bolting the springs in place with bolts53 and 54. The opposite faces of the springs 51 and 52 are secured tothe outer periphery of the plate member 11 by means of nuts 55 and 56that are secured to the studs 57 and 58 which are assembled inproperlylocated tapped holes in plate 11.

In order to very accurately produce a uniform and stable air gap overthe entire surface of the mating magnetic elements, I perform thefollowing operations:

After rigidly bonding the magnetic assemblies 13 and 14 into therecessed portions of plate 11, I grind the exposed ends of the magneticlamination assembly in a plane with the peripheral face of plate 11. Inother words, after the magnetic assemblies are secured in place, plate11 is carefully ground so that its opposite faces are in perfectparallel planes, including the ends of the magnetic laminations. In thenext step of my process to secure a uniform air gap, 1 permanentlyattach the spring members 51 and 52 to the peripheral faces of basemembers 49 and 50 by means of the bolts 53 and 54, as shown. I nextgrind the free unmounted faces of the spring members 51 and 52 so thatthey are in an exact plane with the exposed ends of the magneticlamination assemblies 37 and 38. At this point I will have two matingmagnetic assemblies comprising the inertial portion of the transducercontaining the massive base members 49 and 50 and the spring members 51and 52 and the driven portion of the transducer including the platemember 11 whose opposite parallel surfaces lie in an exact planes withthe free ends of the respective magnetic assemblies. The next step is toassemble the studs 57 and 58 into the peripheral tapped holes in plate11. Then a number of shims 59 and 60 having clearance holes to fit overthe studs are placed between the surfaces of the plate 11 and the facesof the springs 51 and 52. The shims 59 and 60 will exactly determine themagnitude of the air gap which will result, and due to the placement ofthe shims over the entire peripheral surface of the assembledstructures, the air gap will be extremely uniform to result in perfectoperation of the electromechanical vibrating structure. To complete thetransducer assembly, I simply attach the hemispherically-shaped housingstructure 30 and 'a similar structure 62 to the outer peripheralsurfaces of plate 11 by means of bolts 63 and 64. Tapped holes areplaced about the periphery of plate 11 as illustrated in FIG. 3, inorder to receive the bolts 63 and 64. O-ring grooves are provided in theend faces of the housing members 30 and 62, and O-rings 65 are used tocomplete the water-tight seal when the housing structures are attached.Having completed the assembly of the transducer, its operation isclearly obvious from the electromagnetic forces which are generated inthe air gaps when alternating current passes through the coils 21 and22.

My invention has chosen a balanced armature type of electromagnetictransducer, and the polarity of the current and magnets is such that atany given instant a force of attraction is developed at one air gapsurface while a corresponding force of repulsion is developed at theopposite air gap. In this manner the magnetic forces combine so thateffectively the operating area on each side of the plate adds tocontribute to the total electromagnetic driving force and thus twice thedriving force may be generated prior to saturation as would be possibleif only one side of the plate were equipped with a magnetic drive. Itis, of course, possible to eliminate one of the two magnetic assembliesand still take advantage of the teachings of my invention. Byeliminating one of the two symmetrical electromagnetic structures, thepower handling capacity of the transducer will be lowered but all theremaining advantages which have been described in connection with thefabrication of the thin lamination assembly and the adjustment of theuniform air gap as well as the use of 100% active coil will be realized.Also, I have shown two concentric sections of permanent magnet in thearrangement illustrated. In a smaller transducer I could cut theE-shaped crosssectional magnetic structure shown in FIG. 1 along itscenter line and use only one coil and one permanent magnet and stillachieve all the advantages of the teachings of my invention. A singlemagnet and a single coil surrounded by the adjacent laminated magneticstructure forms the minimum active electromagnetic structure that willoperate completely as an electromechanical transducer. The use ofsuccessive concentric elements in the assembly merely increases thetotal effective area of the electromagnetic structure.

It should be obvious to any one skilled in the art that the current incoil 22 should flow in the opposite sense to the current in coil 21 inorder that the magnetic forces will be additive. It is also obvious thatthe common magnetic poles of the permanent magnets should face towardthe center line of the E-shaped assembly, as illustrated by the N-Spolarity symbols in FIGS. 1 and 2.

While there have been shown and described several specific illustrativeembodiments of the present invention, it will, of course, be understoodthat various modifications and alternative constructions may be madeWithout departing from the true spirit and scope of the invention.Therefore, the appended claims are intended to cover all suchmodifications and alternative constructions as fall within their truespirit and scope.

I claim as my invention:

1. In combination in an electromagnetic transducer, two pairs of rigidunitary structures each comprising a first tightly-wound coil assemblyof thin magnetically conducting strip material, a second tightly-woundcoil assembly of thin magnetically conducting strip material having thesame general shape as said first coil assembly and having an internalsurface in substantially uniformly spaced facing relation to the outsidesurface of said first coil assembly t-o define a clearance space,spacing means substantially filling said clearance space, and bondingmeans between said inside and outside surfaces of said coil assembliesand said spacing means, opposite end surfaces of said coil assemblies ofeach rigid unitary structure being machined to lie in parallel planes, arigid circular plate having oppositely facing parallel surface portionsbonded to end surfaces of coil assemblies of one pair of said rigidunitary structures, a pair of base members having planar surfaces bondedto end surfaces of coil assemblies of the other pair of saidrigid-unitary structures, spring members supporting said base membersfrom opposite sides of said rigid circular plate with end surfaces ofthe coil assemblies of said one pair of rigid unitary structures inspaced facing relation to end surfaces of the coil assemblies of saidother pair of rigid unitary structures, a pair of cupshaped housingmembers secured on opposite sides of said rigid circular plate,permanent magnet mean-s forming said spacing means of one pair of saidrigid unitary structures, wound coils of insulated conductors formingsaid spacing means of the other pair of said rigid unitary structures,and terminal means for connecting said wound coils to an externalCircuit.

2. In combination in an electromagnetic transducer, a firsttightly-wound coil assembly of thin magneticallyconducting stripmaterial, a second tightly-wound coil assembly of thinmagnetically-conducting strip material having the same general shape asthe first coil assembly and having an internal surface in substantiallyuniform ly spaced facing relation to the outside surface of said firstcoil assembly to define a clearance space, rigid spacing meanssubstantially filling said clearance space, said spacing means being inthe form of permanent magnetic means, and bonding means between saidinside and outside surfaces of said coil assemblies and said spacingmeans whereby said spacing means and said coil assemblies areconsolidated into a rigid unitary structure.

'3. In an electromagnetic transducer a-s defined in claim 2, said coilassemblies being of generally cylindrical shape, and said permanentmagnet means comprising a plurality of permanent magnets each shaped asa section of a cylindrical shell.

4. In combination in an electromagnetic transducer, a firsttightly-wound coil assembly of thin magneticallyconducting stripmaterial, a second tightly-wound coil assembly of thinmagnetically-conducting strip material having the same generel shape asthe first coil assembly and having an internal surface in substantiallyuniform spaced facing relation to the outside surface of said first coilassembly to define a clearance space, rigid spacing means substantiallyfilling said clearance space, a third tightlywound coil assembly of thinmagnetically conducting strip material having the same general shape assaid first and second coil assemblies and having an internal surface insubstantially uniformly spaced facing relation to the outside surface ofsaid second coil assembly to define a second clearance space, secondspacing means substantially filling said clearance space, and bondingmeans between said inside and outside surfaces of each of said coilassemblies and both of said spacing means, whereby both of said spacingmeans and said three coil assembles are consolidated into said rigidunitary structure.

5. In combination in an electromagnetic transducer, a firsttightly-wound coil assembly of thing magneticallyconducting stripmaterial, a second tightly-wound coil assembly of thinmagnetically-conducting strip material having the same general shape asthe first coil assembly, and having a internal surface in substantiallyuniformly spaced facing relation to the outside surface of said firstcoil assembly to define a clearance space, rigid spacing meanssubstantially filling said clearance space, bonding means between saidinside and outside surfaces of said coil assemblies and said spacingmeans whereby said spacing means and said coil assemblies areconsolidated into a rigid unitary structure, said spacing means havingmagnetic field inducing means for inducing magnetic fields of oppositepoles in said first and second coil assemblies forming a first rigidunitary structure and comprising in addition: third and fourth coilassemblies having sizes and shapes respectively the same as those ofsaid first and second coil assemblies, second spacing means between saidthird and fourth coil assemblies including a wound coil of an insulatedconductor, bonding means between inside and outside surfaces of saidthird and fourth coil assemblies and said second spacing means wherebysaid second spacing means and said third and fourth coil assemblies areconsolidated into a second rigid unitary structure, and supporting meansfor supporting said unitary structures with coplanar faces of said thirdand fourth coil assemblies in spaced facing relation to coplanar facesof said first and second coil assemblies while permitting relativemovement of said rigid unitary structures towards and away from eachother.

6. In an electromagnetic transducer as defined in claim 5, saidsupporting means including spring members se cured to one of saidunitary structures and having end surface portions in the same plane asthe coplanar faces of said assemblies thereof, means secured on theother of said unitary structures and having surface portions in the sameplane as the coplanar faces of said assemblies thereof, and shimsbetween said surface portions for providing certain spacing between saidcoplanar faces of said first and second coil assembly in an unstressedcondition of said spring members.

7. A method of fabricating an electromagnetic transducer comprising thesteps of:

(a) tightly winding a thin strip of magneticallyconducting material toprovide a first coil assembly;

(b) tightly winding around the first coil assembly a thin strip ofmagnetically-conducting material of a width narrower than that of thefirst coil assembly to provide a second coil assembly of narrower widththan the first coil assembly;

(c) tightly winding around the second coil assembly a thin strip ofmagnetically-conducting material of a Width equal to that of the firstcoil assembly to provide a third coil assembly equal in width to thefirst coil assembly;

a slot being formed between the first and third coil as- 8 semblieswhich is bounded on one end by the second coil assembly,

(d) placing a coiled insulated electrical conductor within the slot toform a first rigid unitary structure,

(e) tightly winding a thin strip of magnetically-conducting material toprovide a fourth coil assembly;

(f) tightly winding a thin strip of magnetically-conducting material ofa Width equal to that of the fourth coil assembly to provide a fifthcoil assembly equal in width to the fourth coil assembly but having aninner surface of greater dimensions than the outer surface of the fourthcoil assembly so that when the fourth coil assembly is placed within thefifth coil assembly a clearance space between the fourth and fifth coilassemblies is defined;

(g) placing solid spacing means in the clearance space to provide asecond rigid unitary structure;

(h) preparing a pair of support members having plane surface areas;

(i) machining end surfaces of the coil assemblies of the rigid unitarystructures into coplanar relation; (j) bonding the machined end surfacesof the coil assemblies to the plane surface areas of the supportmembers;

(k) machining opposite end surfaces of the coil assemblies into coplanarrelation; and

(1) connecting the support members to support the opposite end surfacesof the coil assemblies of the rigid unitary structures into spacedfacing relation.

8. A method of fabricating an electromagnetic transducer as described inclaim 7 wherein the step of connecting the support members includes:

(a) securing end faces of spring members to one of the support members;

(b) machining free end faces of the spring members into coplanarrelation with the opposite end surfaces of the coil assemblies securedto the support member having the spring members;

(c) machining end faces of the other support member into coplanarrelation with the opposite end surfaces of the other coil assemblies;

(d) attaching the spring member free end faces to the other supportmember; and

(e) placing shims under the free end faces of the spring members toobtain the proper spacing between the rigid unitary structures.

References Cited UNITED STATES PATENTS 2,160,007 5/1939 Turner 34011 X2,958,078 10/1960 Hickman et a1. 340-8 X 2,962,679 11/1960 Stratton33623 X 3,219,969 11/1965 Snavely 340-8 3,225,326 12/1965 Nlassa 340-83.230502 l/1966 Chervenak 340--8 X 3,260,990 7/1966 Massa 340-l2 RODNEYD. BENNETT, Primary Examiner.

B. L. RIBANDO, Assistant Examiner.

1. IN COMBINATION IN AN ELECTROMAGNETIC TRANSDUCER, TWO PAIRS OF RIGIDUNITARY STRUCTURES EACH COMPRISING A FIRST TIGHTLY-WOUND COIL ASSEMBLYOF THIN MAGNETICALLY CONDUCTING STRIP MATERIAL, A SECOND TIGHTLY-WOUNDCOIL ASSEMBLY OF THIN MAGNETICALLY CONDUCTING STRIP MATERIAL HAVING THESAME GENERAL SHAPE AS SAID FIRST COIL ASSEMBLY AND HAVING AN INTERNALSURFACE IN SUBSTANTIALLY UNIFORMLY SPACED FACING RELATION TO THE OUTSIDESURFACE OF SAID FIRST COIL ASSEMBLY TO DEFINE A CLEARANCE SPACE, SPACINGMEANS SUBSTANTIALLY FILLING SAID CLEARANCE SPACED, AND BONDING MEANSBETWEEN SAID INSIDE AND OUTSIDE SURFACES OF SAID COIL ASSEMBLIES ANDSAID SPACING MEANS, OPPOSITE END SURFACES OF SAID COIL ASSEMBLIES OFEACH RIGID UNITARY STRUCTURE BEING MACHINED TO LIE IN PARALLEL PLANES, ARIGID CIRCULAR PLATE HAVING OPPOSITELY FACING PARALLEL SURFACE PORTIONSBONDED TO END SURFACES OF COIL ASSEMBLIES OF ONE PAIR OF SAID RIGIDUNITARY STRUCTURES, A PAIR OF BASE MEMBERS HAVING PLANAR SURFACES BONDEDTO END SURFACES OF COIL ASSEMBLIES OF THE OTHER PAIR OF SAIDRIGID-UNITARY STRUCTURES, SPRING MEMBERS SUPPORTING SAID BASE MEMBERSFROM OPPOSITE SIDES OF SAID RIGID CIRCULAR PLATE WITH END SURFACES OFTHE COIL ASSEMBLIES OF SAID ONE PAIR OF RIGID UNITARY STRUCTURES INSPACED